Paper machine felts

ABSTRACT

The invention relates to postcondensed paper machine felts comprising a polyamide base fabric and a polyamide coating needled thereon, the paper machine felts having a relative solution viscosity in sulfuric acid in accordance with the DIN 53,727 standard of 5 or more. 
     The invention further relates to a method of increasing the molecular weight of paper machine felts which comprises impregnating the paper machine felts with a solution of postcondensation catalysts, followed by drying and thermally postcondensing the felts below the melting point of the polyamide in a solid phase by the exclusion of oxygen.

BACKGROUND OF THE INVENTION

The invention relates in particular to postcondensed paper machine feltscomprising a polyamide base fabric and a polyamide coating needledthereon.

The invention further relates to a method of increasing the molecularweight of the aforementioned paper machine felts.

Paper machine felts generally comprise a base fabric on which preneedledweb material has been needled. Basically, it is also possible to usespunbonded webs in place of dried web materials.

DE-A-4,027,063 discloses a process for preparing particularlyhigh-weight polyamide fibers by postcondensation. Such postcondensedfibers have the drawback of poor processability because they are veryrigid due to their high molecular weight.

Therefore, more energy is needed for carding and needling, and thisincreased energy enhances the risk of fiber damage during processing.

Another factor to be considered is that postcondensed fibers in the feltcan hardly be heat set, that is to say that tension that builds up inthe fiber during processing cannot be fully eliminated. This promotesfiber shedding, that is the removal of major fiber fragments or evenentire fibers from the felt.

In addition, postcondensed fibers exhibit virtually no thermalshrinkage. The felts are no longer precompressed during the settingprocess necessary for the base fabric. As a result, fiber bonding maynot be optimal.

It is therefore the object of the invention to provide paper machinefelts having a high resistance to chemicals, high air permeability andimproved wear resistance.

This object is achieved by the postcondensed paper machine felts definedin claim 1 and by the method defined in claim 6. The subclaims containadvantageous embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the method by which the felts used in thecomparative experiments described in the present application were made.

FIG. 2 is a schematic view of the pressure test procedures utilized inthe comparative examples described in the present application.

FIG. 3 is a schematic view of the abrasion test used in the comparativeexamples described in the present application.

DETAILED DESCRIPTION OF THE INVENTION

It is not a matter-of-course for someone skilled in the-art that thereis a difference in quality between paper machine felts comprisingpostcondensed fibers as known in the state of the art and postcondensedpaper machine felts as defined in the present invention.

Yet, simultaneous postcondensation of the base fabric comprisingmonofilaments and/or multifilaments is expected to result-in a certainadvantage. In general, however, the resistance of the base fabric is notproblematic.

However, it has been found that, surprisingly, tests conducted on felttesting presses (see also Table 1 below) revealed significantdifferences between standard felts and postcondensed paper machine feltsas defined in the present invention. When compared to felts comprisingpostcondensed fibers, the postcondensed felts of the present inventionshowed a clearly lower change in air permeability, with the final valuesfor both felts being similar, however. This is advantageous in themanufacture of paper because it causes the startup time to be shorterand the felt properties to undergo only slight changes during thestartup time.

It has also been a surprising finding that the two felts considerablydiffered with respect to fiber loss.

On the whole, it has been found that, surprisingly, postcondensed papermachine felts as defined in the present invention will have the requiredgood resistance to chemicals and abrasion if they have a solutionviscosity of 5 or more as determined in sulfuric acid at 20° C. (inaccordance with the DIN 53,727 standard).

The polyamide fibers of the paper machine felts postcondensed by usingthe methods of the present invention comprise in particular aliphatic orpartly aromatic polyamides or copolyamides, the aliphatic polyamides orcopolyamides being based on m-amino carboxylic acids, lactams oraliphatic diamines and aliphatic dicarboxylic acids having 4 to 12carbon atoms, and the partly aromatic polyamides or copolyamides beingbased on aliphatic monomers having 4 to 12 carbon atoms. Among them,polyamide 4, polyamide 6, polyamide 11, polyamide 12, polyamide 46,polyamide 66, polyamide 610, polyamide 612, polyamide 1212, polyamide10T and polyamide 12T are preferred.

Examples of postcondensation catalysts include inorganic phosphoruscompounds, preferably salts or esters of phosphoric acid or orthophosphoric acid, or such acids themselves, with H₃ PO₄, H₃ PO₃, Na₂HPO₄.12H₂ O, Na₂ HPO₃.5H₂ O and NaH₂ PO₄ being more preferred. Thetextile fabrics are impregnated, the content of catalyst of thepreferably aqueous solution being no higher than 0.5% by wt., preferably0.1 to 0.3% by wt., more preferably 0.2% by wt., based on the amount oftextiles to be postcondensed. Postcondensation is conducted in an inertgas atmosphere or under vacuum at temperatures between 160° and 200° C.,preferably between 170° and 190° C., for 5 to 48 hours, preferably 6 to24 hours, more preferably 8 to 12 hours.

In a particularly advantageous embodiment of the method of the presentinvention the textile fabric is postcondensed with aqueous solutions ofH₃ PO₄ or H₃ PO₃ in amounts of 0.2% by wt., based on the amount oftextiles to be postcondensed, at 180° C. under vacuum for 8 hours.

The paper machine felt of the present invention comprising polyamidefibers has a relative solution viscosity, determined as a 1% solution in98% sulfuric acid (DIN 53,727), of 5 or more, preferably 6 or more, morepreferably 6.5 or more, most preferably 7 or more. The polyamide fibersare in particular such comprising m-amino carboxylic acids or lactamshaving 4 to 12 carbon atoms or such comprising aliphatic diamines andaliphatic dicarboxylic acids having 4 to 12 carbon atoms.

Among them, polyamide 4, polyamide 6, polyamide 11, polyamide 12,polyamide 46, polyamide 66, polyamide 610, polyamide 612 and polyamide1212 are preferred.

Another embodiment includes partly aromatic polyamides or copolyamidescomprising aliphatic monomers having 4 to 12 carbon atoms and aromaticmonomers having 6 to 12 carbon atoms, in particular polyamide 10T andpolyamide 12T.

A particular advantage of the present invention is the fact that it ispossible to first produce textile fabrics from polyamide fibers havinglow viscosity and being easy to process in a manner known per se withoutcausing fiber damage and then increase their molecular weight bypostcondensation to a relative solution viscosity in sulfuric acid of 7or more, while increasing crystallinity and setting the form of thetextile fabrics at the same time.

The following examples illustrate the embodiments of the inventionwithout being limitative.

EXAMPLE 1 Postcondensation of paper machine felts

A piece of paper machine felt of 1×0.5 m in size consisting of a basefabric comprising polyamide 6 monofilaments (nrel=3.4±0.1) and a webneedled thereon as a coating comprising polyamide 6 fibers (GrilonRTM26R, nrel=3.4±0.1, determined as a 1% solution in 98% sulfuric acid inaccordance with the DIN 53,727 standard at 20° C.) was impregnated withan aqueous solution of phosphoric acid (0.2% by wt., based on the weightof the felt). Upon drying in the air, the felt was postcondensed in alaboratory autoclave under vacuum at 180° C. for 16 hours. The solutionviscosity of the resulting postcondensed paper machine felt in sulfuricacid was 10.5±0.5.

EXAMPLE 2

A paper machine felt of 2×0.2 m in size consisting of a base fabriccomprising polyamide 6 twists (monofilaments) (nrel=3.4), and a webneedled thereon as a coating comprising polyamide 6 fibers (GrilonRTM262R, 17 dtex, 90 mm) was impregnated with an aqueous solution ofphosphoric acid (0.24%) in a dyeing autoclave at 98° C. for 30 minutes.Then the felt was dried at 60° C. for 18 hours. Postcondensation wasconducted in a vacuum furnace at 180° C. for 16 hours. The analyticaldata of this sample (sample 2) are shown in Tables 1 and 2.

Comparative Examples

Sample 3 consists of a felt comprising TM262R.

Sample 4 consists of a felt comprising TM262R, with the fibers havingbeen postcondensed (30 minutes, 98° C.; 16 hours, 180° C., vacuum) andthe relative viscosity of the fibers being 7.8.

                                      TABLE 1                                     __________________________________________________________________________    Thickness of Felt Air Permeability                                                Unset         Unset         Fiber Loss                                    Sample                                                                             m/m!                                                                             Set                                                                              After Testing                                                                         1/m.sup.2 s!                                                                     Set                                                                              After Testing                                                                         g/m.sup.2 !                                  __________________________________________________________________________    2   5.13                                                                              5.19                                                                             2.86 = 55.1%                                                                         374 375                                                                              53 = 14.1%                                                                           21                                            3   5.09                                                                              5.22                                                                             2.60 = 49.8%                                                                         500 418                                                                              38 = 9.1%                                                                            30                                            4   5.26                                                                              5.26                                                                             2.79 = 53.0%                                                                         502 507                                                                              55 = 9.6%                                                                            26                                            __________________________________________________________________________

Experimental Conditions

For the experiments, three felts were produced as shown in FIG. 1.Samples 3 and 4 were regarded as standard felts and felt 2 was treatedas follows:

The felt was impregnated with a 0.24% acidic solution in a dyeingautoclave at 98° C. for 30 minutes. Then the felt was dried at 60° C.for 18 hours. Postcondensation was conducted in a vacuum furnace at 180°C. for 16 hours (see example 2).

Analysis and Analytical Results

The relative viscosities of the fibers and monofilaments were determinedin 1% sulfuric acid.

                  TABLE 2                                                         ______________________________________                                                                      Mono-                                                                         filament                                                                            Monofilament                              Sample               Fibers   gray  white                                     ______________________________________                                        3     Standard Felt  η.sub.rel = 3.3                                                                    3.4   3.4                                       2     Postcondensed Felt                                                                           η.sub.rel = 6.6                                                                    7.3   8.11                                      4     Standard Felt Comprising                                                                     η.sub.rel = 7.8                                                                    3.4   3.4                                             Postcondensed Fibers                                                    ______________________________________                                    

Felt Testing Press

The felts were tested on the FTP-EMS felt testing press.

In the test a sample felt of 2×0.2 m in size was locked in two colletchucks. The collet chucks were connected by a rope beneath the machineand were pulled back and forth during the test. The test comprised thepartial steps of pressure test, pressure test including high-pressureshowers and abrasion test. In the pressure test the felt was moved backand forth by means of a pair of press rolls (FIG. 2a). During the courseof the test, the felt was constantly wetted before and after the rollslit. The pressure along a line of the pair of press rolls wasadjustable between 0 and 300 kN/m. To measure the compression of thefelt, thickness and air permeability were determined after differentpressing processes.

In the pressure test including high-pressure showers (HP showers) thefelt was wetted with an oscillating high-pressure shower (waterpressure: 40 bars) before and after the roll slit (FIG. 2b). Theinfluence of the HP shower was evaluated optically and the fibers thathad been removed and collected in a filter were weighed.

In the abrasion test including ceramic bars a ceramic bar imitation rollwas used (FIG. 3). Slits were cut crosswise on the roll so that theremaining webs took the form of suction bars. During the test the feltsample was pulled back and forth by the rope control beneath thefast-moving abrasion roll. The resistance of the felts to abrasion wasevaluated microscopically and by measuring the amount of worn fibers.

Test Steps

A. washing and setting

B. 100×press rolling (PR) at a pressure along a line of 150 kg/cm

C. +2700×PR=2800×PR

D. 200×high-pressure showering (HS) using a water pressure of 40 barsand press rolls at a pressure of 150 kg/cm

E. +800×HS=1000 HS

F. 500×abrasion rolling.

Using a sample, treatments A to F were conducted sequentially. Then feltthickness, air permeability and fiber loss were determined and comparedto the untreated sample.

Results

Table 1 shows the results of the samples treated with the felt testingpress.

The thickness of the postcondensed felt (sample 2) is least-affected bythe test. Sample 2 has the largest thickness after the test.

The air permeability of the standard felts (samples 3 and 4) is higherthan that of the postcondensed felt (sample 2) both in the unset and setstates.

The change in air permeability caused by the treatment in the felttesting press is the lowest in the postcondensed felt (sample 2), thatis, sample 2 has the most uniform properties over the entire testperiod.

At 30 g/m² (sample 3) and 26 g/m², the fiber loss of the comparativefelts is clearly higher than that of the postcondensed felt (sample 2,21 g/m²).

What is claimed is:
 1. Paper machine felts having decreased in-use fiberloss and decreased in-use change in air permeability comprising apolyamide base fabric and a polyamide coating needled thereon, said basefabric and coating being postcondensed after formation, and said papermachine felts having a relative solution viscosity in sulfuric acid inaccordance with the DIN 53,727 standard of 5 or more.
 2. Paper machinefelts as defined in claim 1 having a relative solution viscosity insulfuric acid of 7 or more.
 3. Paper machine felts as defined in claim 1wherein the polyamide is an aliphatic polyamide or copolyamidecomprisingamino carboxylic acids or lactams having 4 to 12 carbon atoms.4. Paper machine felts as defined in claim 1 wherein the polyamide is analiphatic polyamide or copolyamide comprising aliphatic diamines andaliphatic dicarboxylic acids having 4 to 12 carbon atoms.
 5. Papermachine felts as defined in claim 1 wherein the polyamide is a partlyaromatic polyamide or copolyamide comprising aliphatic monomers having 4to 12 carbon atoms and aromatic monomers having 6 to 12 carbon atoms. 6.A method of increasing the molecular weight of paper machine felts whichcomprises starting with a non-postcondensed paper machine felt,impregnating said paper machine felt with a solution of postcondensationcatalysts, followed by drying and thermally postcondensing the feltbelow the melting point of the polyamide in a solid phase by theexclusion of oxygen.
 7. The method as defined in claim 6 wherein thepostcondensation catalysts are inorganic phosphorus compounds.
 8. Themethod as defined in claim 7 wherein the postcondensation catalysts areapplied on the paper machine felt in the form of aqueous solutions. 9.The method as defined in claim 7 wherein the amount of catalyst is nohigher than 0.5% by wt. based on the amount of paper machine felt to bepostcondensed.
 10. The method as defined in claim 7 whereinpostcondensation is conducted in an inert gas atmosphere or under vacuumat temperatures between 160° and 200° C., preferably 170° and 190° C.11. The method as defined in claim 7 wherein postcondensation isconducted over a period of 5 to 48 hours.
 12. The method as defined inclaim 7 wherein the paper machine felt is postcondensed with aqueoussolutions of H₃ PO₄ or H₃ PO₃, 0.2% by wt., based on the amount of papermachine felt to be postcondensed, at 180° C. under vacuum for 8 hours.13. Paper machine felts as defined in claim 1 wherein the polyamide isselected from the group consisting of polyamide 4, polyamide 6,polyamide 11, polyamide 12, polyamide 46, polyamide 66, polyamide 610,polyamide 612, polyamide 1212, polyamide 10T, polyamide 12T, andmixtures thereof.
 14. The method as defined in claim 7 wherein thepostcondensation catalysts are selected from phosphoric acid, orthophosphoric acid, and salts and esters thereof, and the postcondensationis conducted over a period of 6 to 24 hours.